A curveball is a batter’s enemy in baseball. The ball leaves the pitcher’s hand in a fast straight pitch, while the batter gets into stance and keeps their eye on the ball.

Then just before the ball reaches the batter, the ball appears to break in a jarring change of direction. Major league pitchers can throw master curveballs that veer as much as 17 inches from a straight line by the time it passes the homeplate. Batters report the dastardly pitches shift at the last minute or even appear to rise when the ball is actually falling.

Pitchers are indeed talented, but they may not be the secret behind why curveballs are so tricky. Instead, curveballs succeed by tricking the human peripheral system, according to a recent study published in online science journal PLoSONE.

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The curveball never breaks, but instead follows a gradual and steady parabolic path. Our eyes just have trouble handling it.

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Batters will use their foveal, or central, vision when the ball leaves the pitcher’s hand. About 20 feet in, the batter will switch to peripheral vision, which occurs outside of the very center of the gaze. By the time the ball crosses home plate, the batter reverts back to central vision. Humans constantly switch focus like this.

The problem is peripheral vision processes first and second order motion in conjunction, while foveal vision distinguishes between the two.

During the peripheral stage, batters do not notice that the curveball has been gradually changing direction ever since it left the pitcher’s mound.

Once they switch back to the foveal vision, batters accurately register the changed direction, but consider it a result of the ball’s immediate “break,” instead of their temporary impaired vision.

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It makes the coach’s adage to “keep your eye on the ball” nearly impossible. Not only do we switch vision, the visual processing we use while following the ball can’t process curveball mechanics.

The researchers generated an Adobe Flash interactive that mirrored the physics of a curveball and observed the responses of five participants who watched the digital curveballs.

The study authors, from the Psychology Departments of American University and USC and from the Mayo Clinic and SUNY College of Optometry, believe their results have deeper implications for how humans process visual stimuli, especially in designing aircraft landing strips and building layouts.

Photo: San Francisco Giants pitcher Madison Bumgarner pitches during the first inning of Game 4 of baseball’s National League Division Series against the Atlanta Braves on Monday, Oct. 11, 2010, in Atlanta. (AP Photo/John Bazemore)